The influence of surface oxidation on the pH-sensing properties of siliconnitride

The pH-sensing properties of silicon nitride were studied with special emph azise on surface oxidation. Simulations of the surface potential and the se nsitivity were performed using the site binding theory to show the influenc e of an oxygen rich surface and surface oxidation of silicon nitride. Sampl es were produced by depositing silicon nitride in a LPCVD process with how ratios of the processing gases NH3 and SiCl2H2 ranging from 2 to 20. The sa mples were characterized by XPS and capacitance-voltage measurements of EIS -structures. Silicon nitride etched in buffered HF before measurement showe d a pH-sensitivity of up to 58 mV pH(-1), and an increasing hysteresis was found with increasing NH3/SiCl2H2 gas-flow ratios during deposition. Sample s not etched had only a pH-sensitivity between 45 and 52 mV pH(-1) because of their oxygen rich surface layer. The oxygen rich layer was found to be a pprox. 4 nm deep and soluble in buffer electrolyte. Surface oxidation and d issolution of the oxygen rich layer is claimed to be decisive for the pH-se nsing properties of silicon nitride. (C) 1999 Elsevier Science S.A. All rig hts reserved.